The two-decade search for an AIDS vaccine is in crisis after two field tests of the most promising contender not only did not protect people from the virus but may actually have put them at increased risk of becoming infected, The Washington Post reported.

Experts are questioning the overall strategy and scientific premises of the nearly $500 million in AIDS vaccine research funded annually by the government after the two field tests were halted last September and seven other trials of AIDS vaccines have either been stopped or put off indefinitely.

The recently closed studies, STEP and Phambili, were halted when it became clear the STEP study was futile and possibly harmful.

The hypothesis for what went wrong in a scenario unforeseen by previous studies is that the vaccine managed to prime the immune system to be more susceptible to HIV infection.

"We simply do not know at the present time how to design a vaccine that will be effective against HIV," Ronald C. Desrosiers, a molecular geneticist at Harvard University said at an AIDS conference last month.

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